The scale on a map is 1cm : 10km. If the distance between two cities is 100km, how far apart in cm are the two cities on the map?
Explanation: The scale means that every 1cm on the map corresponds to an actual distance of 10km. An actual distance of 100km is the same as 10 $\cdot$ 10km. The distance between the two cities on the map is 10 $\cdot$ 1cm, or 10cm.